The invention relates to a P-N Diode suitable for use as a rectifier and a method of making it. In particular, the invention provides a fast bipolar P-N diode of the type having an intermediate drift region, i.e. of the P-i-N type.
P-i-N rectifiers are well known for power circuit applications. An intermediate drift region (i-region) is sandwiched between highly doped p and n regions of a junction diode. The intermediate drift region has a lower doping than the p or the n regions. During forward conduction, the i-region is flooded with minority carriers therefore allowing the resistance of the region to become very small to allow the diode to carry a high current density during forward conduction. A useful discussion of such devices is contained in the textbook xe2x80x9cPower Semiconductor devicesxe2x80x9d by B. Jayant Baliga published by the PWS Publishing Company, Boston in 1995, particularly section 4.2, pages 153 to 182.
With suitable choices of parameters, especially the thickness and dopant concentration of the i-region, it has proved possible to develop such rectifiers with very high breakdown voltages
Significant drawbacks of the P-i-N rectifier relate to the need to inject a high concentration of minority carriers into the i-region for conduction. This gives rise to two problems. Firstly, as the diode is turned on the diode has a very high resistance until the minority carriers are injected into the i-region. Accordingly, during turn-on there is a high current flow across a resistive region which gives rise to a large voltage drop across the diode.
Secondly, during turn-off the minority carrier charge stored in the i-region must be removed. This requires a significant time, a large reverse current together with a negative voltage overshoot. As the switching speed of a diode or rectifier increases, these effects become worse. In particular, it would be beneficial to reduce the reverse recovery time.
It has been found that a faster reverse recovery can be obtained when the carrier lifetime in the i-region is reduced. Accordingly, a popular approach to speeding reverse recovery is to include gold or platinum recombination centres in the i-region. This is known as gold or platinum xe2x80x9ckillingxe2x80x9d and has been found to significantly improve reverse recovery times.
Unfortunately, gold or platinum killed diodes suffer from a high temperature dependence of their switching behaviour, since minority carrier lifetime increases rapidly with temperature when controlled by deep recombination centres such as gold and platinum. Furthermore, the carrier distribution in a heavily gold or platinum doped i-region is uneven across the thickness of the i-region. This is because the short carrier lifetime means that recombination reduces the carrier density in the middle of the i-region. This U-shaped carrier distribution leads to a non-soft recovery and to ringing, because the higher carrier density at the edges of the i-region takes longer to fall to zero than the central region which accordingly becomes suddenly depleted during the application of blocking voltage giving a rapid decay of current leading to snap-off and ringing.
Accordingly, there is a need to improve the performance of the P-i-N diode structures for rapid-switching high-power applications.
According to the invention there is provided a semiconductor diode, comprising a first doped region of a first conductivity type; an intermediate drift region of the first conductivity type doped with a dopant concentration less than that in the first region; a second doped region of a conductivity type opposite to the first conductivity type, sandwiching the intermediate drift region between the first and second regions, wherein the second doped region forms a bipolar diode structure with the intermediate and first regions; and a plurality of field electrodes for depleting the intermediate drift region under reverse bias, the plurality of field electrodes being arranged in a plurality of closely spaced insulated trenches extending laterally across the semiconductor diode and extending through the second and intermediate regions.
As is explained in more detail below, the use of this structure permits a significantly improved recovery time.
For a given dopant concentration for the intermediate drift region, the use of the field electrodes increases the breakdown voltage that would otherwise be obtained under reverse bias, since in this situation the depletion layer around the trenches and between the opposite conductivity type regions of the bipolar diode extends so far as to deplete the whole intermediate region. Under forward bias, the depletion layer retreats to allow conduction through the diode structure.
However, more importantly, the use of the structure according to the invention allows a higher than normal doping of the intermediate drift region for a given required blocking voltage. The doping may be of the same order of magnitude as the minority carrier density injected into the drift intermediate drift region. Accordingly, the intermediate drift region may have a dopant concentration from 1015 cmxe2x88x923 to 1017 cmxe2x88x923.
The use of higher doping in the intermediate drift region reduces charge storage in the drift region during forward conduction since much of the charge required to carry current is obtained from dopant atoms in that region rather than being injected by minority carriers. Thus, the level of injected charge needed for a given current is reduced. This greatly increases the speed of reverse recovery without needing lifetime killer centres such as gold and platinum doping. Accordingly, the disadvantages of non-soft recovery and high temperature dependence of switching characteristics that would be associated with such doping can be reduced or eliminated.
Furthermore, the use of a higher doping gives rise to a lower transient voltage drop during turn-on. During turn-on, there is normally a larger voltage drop across a conventional p-i-n diode than in the steady-state, because the injected charge distribution has not yet been fully established in the intermediate region. The higher doping level allowed by the invention can lower the pre-modulation resistance and reduce the voltage overshoot. This effect is additional to the reduced reverse recovery time. Thus, the invention can produce advantageous results both during turn-on and turn-off.
The size of the improvement in recovery time will vary with a number of factors, such as the distance between trenches.
The more closely spaced the trenches, the greater the improvement will be, but as a trade-off the more silicon real estate will be used for the trenches.
Accordingly, the skilled person will select the distance between the trenches to be sufficiently closely spaced to give a benefit in recovery time without sacrificing too much area. Generally, the trenches will be at a spacing of less than 10 xcexcm.
There may be no lifetime killers in the intermediate drift region. Alternatively, there may be a reduced density of lifetime killers compared with prior art diodes. In this way, the temperature dependence of diode properties may be reduced.
A similar structure of field regions is used in Schottky diode structures, such as described for example in U.S. Pat. No. 4,646,115, (our ref. PHB33047). In the structure described in that document field relief regions are provided to improve the breakdown voltage of the main junction to a value higher than would be possible without the field relief regions. This in turn allows the use of a higher doped (and possibly thinner) epilayer than would normally be allowed for a particular Schottky breakdown voltage. This is of special benefit in unipolar devices since the doping and thickness of the epi-layer has a large influence on the on-state voltage drop which may accordingly be reduced. As the skilled person will appreciate, the ability to use a higher doping level than normally allowed for a given blocking voltage has very little influence on the on-state voltage drop of a bipolar, p-n diode. Accordingly, little or none of the benefit of the field regions that would be obtained in a Schottky device is obtained in bipolar devices and accordingly field relief regions have not, as far as the inventor is aware, been used on bipolar diode structures.
In a p-n diode in accordance with the present invention, the second doped region and intermediate drift region may be formed as epitaxial layers (also termed epilayers) deposited on a substrate. The first doped region may likewise be an epilayer or may alternatively be constituted by the doped substrate. The trenches preferably extend into the first doped region.
The field electrode of the trench may be of polycrystalline silicon (also termed polysilicon) doped to the same conductivity type as the second doped region.
The first doped region may be a highly doped n+layer, the intermediate drift region may be an n layer and the second doped region may be a p layer.
The field electrode may be of any suitable conductor, such as doped polysilicon.
The insulated trenches and field electrodes may form a grid extending across the semiconductor device.
A top metallisation may be provided extending across the semiconductor diode to contact the field electrodes and the second doped region. A rear contact metallisation may be arranged on the opposite face of the semiconductor diode to the top metallisation to provide an electrical connection to the first doped region.
In embodiments, the first doped region and intermediate drift region may be formed of a first semiconductor material and the second doped region may be a thin region formed of a second semiconductor material having a lower band-gap than the first semiconductor material. The lattice mismatch of the first and second semiconductor materials and the thickness of the thin second region may be selected such that the level of mechanical stress remains below a level at which misfit dislocations are formed. By using such a structure, the doping required in the intermediate drift region may be higher than otherwise for a given blocking voltage, which in turn may improve the properties of the diode as explained above.
Preferably, the product of the thickness of the thin second region and the relative deviation of the lattice constants of the first and second semiconductor materials does not exceed 30 nm %, to avoid excessive misfit dislocations.